U.S. Pat. No. 5,650,054 describes the structure of a proton conductor gas sensor. In this gas sensor, a lower part of a metal can is used as a water reservoir, and the body of a CO sensor is held in an upper part thereof with a necked part in between. The body of the CO sensor comprises an MEA comprising a proton conductive membrane and electrodes on the front face and the back face thereof, and porous carbon sheets for gas dispersion also serving as electron-conductive contacts. The bottom of the sensor body is made to contact a metal washer having a water vapor introducing hole, and the upper part thereof is made to contact a metal washer having a dispersion control hole. The washer at the bottom is held by a constricted part of the metal can, a gasket is provided between the upper washer and the metal can, the upper part of the metal can is narrowed down to fit the upper and lower metal plates (washers) and the sensor body held between them in the metal can. The sensor body is held by the pressure due to narrowing, and connection between the upper and lower metal plates and the carbon sheets of the sensor body is also maintained by the pressure.
Self-diagnosis of proton conductor gas sensor is proposed by, for example, U.S. Pat. No. 6,200,443. Weak voltage pulses are applied between the sensing electrode and the counter electrode of a gas sensor for a short time, and the capacitance after turning off the pulses is measured. It is claimed that the capacitance of a normal sensor and that of a defective sensor differ from each other, hence self-diagnosis can be done. However, as the internal resistance of electrochemical gas sensors is low, being ohm order. When a voltage is applied between the two electrodes, even if they are weak pulses for a short period, there are possibilities of changes in the electrolyte or the electrodes and generation of hysteresis. Moreover, a pulse power source is needed for self-diagnosis, and detection can not be done until elimination of the hysteresis after application of pulses.    Document 1: U.S. Pat. No. 5,650,054    Document 2: U.S. Pat. No. 6,200,443